Unusual quantum states may shake up quantum computing

Researchers have found a new method of controlling the quantum states of atoms …

Researchers have found a new method of controlling the quantum states of solid particles, and the research could enable a different approach to quantum computing, according to a paper published in Nature.

Using doped silicon, scientists found that they were able to exert control over the solid atoms using terahertz frequency radiation, getting the atoms to oscillate between states normally found in hydrogen atoms. While the equipment they used was very specialized, the authors hope that this new level of coherent control will allow for a different style of entanglement, as well as finer manipulation of quantum information held in excited atoms.

Scientists have had a field day using lasers to perform all manner of quantum manipulations on various particles: trapping photons, entangling them, and sending them over long distances, even using this to perform simple quantum calculations. However, these quantum states are often unstable and hard to control, creating errors and unreliability in their information and in the calculations they perform.

To help fix this, a group of researchers are pursuing the use of well-known quantum states, called Rydberg states, using atoms in a solid. These states follow the rules of a formula that was designed to explain free hydroden atoms, but a special physical loophole allows larger atoms to be obey the formula, too.

Atoms that can be manipulated in Rydberg states have an interesting property: their ground states are too compact to interact with each other. If two such atoms were used for quantum computing, they could be entangled only in their excited states, leaving their ground states independent. This could provide a new quantum information relay method.

A material's impurities can occupy Rydberg states if they have exactly one more valence electron than the host material. Phosphorus-doped silicon, a common type of semiconductor, fits the bill perfectly. The researchers behind the new paper decided they would use radiation in the terahertz range on the phosphorus impurities, hoping that the fine oscillations would let them switch the atoms between two closely spaced Rydberg states.

In order to prove they were actually exerting control over the phosphorus at a quantum level, the researchers had to look for two kinds of activity in the atoms. One was Rabi oscillations, a set of waves that indicate the atom is being driven between a ground and an excited state by the laser. If the laser was set to the right frequency, it would create a superposition of the ground and excited wavefunctions that emits a simple, easily detected wavepacket that oscillates over time.

The other thing they needed to find was photon echoes from the particles. An echo comes from putting a set of particles into a coherent state by zapping them with a laser, then letting their wave functions evolve over time. A second laser pulse would reverse the evolution and cause the particles to send out an extra "echo" of energy.

Packed into the echo is a record of the environment's effects on the atom—a sort of "while you were away" note to would-be observers. Reading the echo would allow researchers to figure out exactly how long the wavefunctions of the atoms would take to "dephase," or get all confused by environmental effects. That length of time is a stand-in measure for how resilient the wavefunctions are, and how long they could be used to hold or transfer quantum information.

Using the Free Electron Laser for Infrared Experiments (FELIX) in the Netherlands, the researchers were able to create laser pulses that exerted remarkable, and remarkably fast, control over the phosphorus atoms. At terahertz frequencies, they were able to stimulate both Rabi oscillations and photon echoes. The Rabi oscillations indicated a superposition of the atoms' ground and first excited states, and had energies that corresponded to the Rydberg series they were looking for.

The photon echoes that the atoms generated indicated that they had a dephasing time of 160 picoseconds. This sounds too short to get much of anything accomplished, but the electrons in the phosphorus atoms were oscillating between states every 100 femtoseconds. If the atom were carrying information, theoretically this means users have over a thousand opportunities to read the atom before its wavefunction gets too distorted to use.

Going forward, the authors of the paper seemed most interested in the prospects for entangling pairs of Rydberg-capable impurities. A Bell state where each atom's ground state is entangled with the other's excited state sounds particularly interesting, since reading one of the atoms has only a chance of setting the other. Aside from that, the researchers hope their new method for exerting coherent control over the quantum states of a solid, widely used material will introduce a bit more versatility to the field of quantum computing.